Conduit passages are commonly formed in cast concrete beams by positioning a knock-out member in the form used to cast the beam, filling the form with concrete, allowing the concrete to harden and then knocking the member out of the resultant beam to form the passage. The members are conventionally made of wood or styrofoam. The openings formed by the members are used for stringing utility conduits through the thickness of the beam.
Problems are encountered using wooden and styrofoam knock-out members. Wooden members absorb water, tend to crack and have a limited useful life. The walls of passages formed using wooden knock-out members are rough and can injure conduits which are pulled through the passages.
The styrofoam knock-out members are easily removed from the concrete beam. Removal of the styrofoam members destructs the styrofoam, forming a large number of relatively small styrofoam beads which tend to stick to the concrete and may become scattered at the removal site. These styrofoam fragments, together with the main body of the member, must be collected and disposed of in an environmentally-safe manner.